The search for a non-addicting narcotic analgesic has served as a focus for opiate pharmacology research for nearly 50 years. The discovery of enkephalins has led to the synthesis of a number of potent analogs with in vivo pharmacologic activity. These analogs represent a new class of narcotic drugs. From the viewpoint of drug development, it is important to know the relative addictive liabilities of one enkephalin analog to another, because addictive liability is one of the principal adverse effects of opioid drugs. In my laboratory, a novel method of drug delivery, the osmotic minipump, was used to measure and compare the activities of eight enkephalin analogs in producting physical dependence in the rat. I found that there was a positive rank correlation between the short-term activities of eight enkephalin analogs in inhibiting a motor response to a noxious stimuli and the long-term activities of the enkephalin analogs in producing physical dependence. The methods developed in my laboratory may be useful for evaluating the dependence potential of new opioid peptides that are now being discovered (e.g. beta-casomorphin, kyotorphin, dynorphin) and in studies aimed at clarifying the mechanisms that underlie physical dependence and addiction.